Method for extending a network of existing fractures

ABSTRACT

The present invention relates to a method for extending a network of existing fractures in a subterranean formation. This method comprises pumping a first fluid into a pressurized volume to pressurize a compressible fluid in said pressurized volume to a predetermined pressure and releasing a second fluid out of the pressurized volume in a confined volume connected to the existing fractures, the second fluid being released due to a relaxation of the pressurized compressible fluid in the pressurized volume. Moreover, the release of second fluid exerts pressures in the existing fractures exceeding a fracturing threshold of the subterranean formation.

PRIORITY CLAIM

The present application is a National Phase entry of PCT Application No.PCT/EP2012/064720, filed Jul. 26, 2012, which claims priority from U.S.Patent Application No. 61/515,058, filed Aug. 4, 2011, said applicationsbeing hereby incorporated by reference herein in their entirety.

FIELD OF THE INVENTION

The present invention relates to method for fracturing of rocks ingeological formations.

BACKGROUND OF THE INVENTION

The approaches described in this section could be pursued, but are notnecessarily approaches that have been previously conceived or pursued.Therefore, unless otherwise indicated herein, the approaches describedin this section are not prior art to the claims in this application andare not admitted to be prior art by inclusion in this section.Furthermore, all embodiments are not necessarily intended to solve allor even any of the problems brought forward in this section.

In order to increase or restore the rate at which fluids, such aspetroleum, water, or natural gas can be produced from subterraneannatural reservoirs, one may need to fracture rocks in subterraneanformation.

Reservoirs are typically porous sandstones, lime stones or dolomiterocks, but also include “unconventional reservoirs” such as shale rockor coal beds. Rock fracturing enables the production of natural gas andoil from rock formations deep below the earth's surface (generally,deeper than 1,000 m).

At such depth or in particular configurations, there may not besufficient permeability or reservoir pressure to allow natural gas andoil to flow from the rock into the wellbore at economic rates. Thus,creating conductive fractures in the rock is useful to extract gas fromshale reservoirs because of the extremely low natural permeability ofshale. Fractures provide a conductive path connecting a larger volume ofthe reservoir to the well.

In order to fracture the rock, one may use:

-   -   “Pulse(s) fracturing” techniques: these techniques allow        fracturing by generating pressure pulse(s) applied to the        geological formation. These pulses me be generated by the        release of a shot, consumable switch, a burn through diaphragm        or an explosive valve. For instance, it may involve the use of        electric line or tubing-conveyed tools to ignite a propellant        charge, similar in composition to solid rocket fuel, which is        positioned across the formation. The propellant burns within a        few milliseconds and creates a high-pressure gas pulse. These        techniques are usually known as “extreme overbalance”;    -   “Hydraulic fracturing” techniques: these techniques allow        fracturing rocks by pumping a fluid into a wellbore at a        sufficient rate to increase the downhole pressure. Thus, the        downhole pressure exceeds the pressure of the failure of the        rock. In these techniques, the operators often inject proppant,        a material such as grains of sand, ceramic, or other        particulates, that prevent the fractures from closing when the        injection is stopped and the pressure of the fluid is reduced.

These techniques are widely in use, but mainly aimed at creating afracture network where none is present, and have some importantlimitations. Such methods have drawbacks.

Examples and possible embodiments of the prior art may be found in U.S.Pat. No. 5,271,465.

The “pulse” techniques do not allow an accurate control of the createdfractures and the extent of the created fractures is limited by theenergy contained in the pulse used. Even in case of a plurality ofpulses, the small energy of each pulse limits the ability to create longfractures.

The “hydraulics fracturing” techniques need important pumping means atthe surface to increase and maintain for several hours the pressure inthe borehole so that the pressure exceeds the pressure of the failure ofthe rock. Moreover, the control of the pressure may be difficult in suchtechniques. Finally, the use of proppant (including a lot of chemicaladditives) is considered as polluting. The use of proppant also inducesa number of constraints such as a control of a constant flux pump rate(flow velocity), of the pH of the carrier fluid and of various otherrheological factors. For instance, the use of industrial water is, inpractice, mandatory to validate these constraints.

There is thus a need for allowing the extension of a multidirectionalfracture network from an existing initial fracture network with minimalpumping equipment at the surface and with minimum impact on theenvironment.

SUMMARY OF THE INVENTION

The invention relates to a method for extending a network of existingfractures in a subterranean formation, comprising:

-   -   /a/ pumping a first fluid into a pressurized volume to        pressurize a compressible fluid in said pressurized volume to a        predetermined pressure;    -   /b/ releasing a second fluid out of the pressurized volume in a        confined volume connected to the existing fractures, the second        fluid being released due to a relaxation of the pressurized        compressible fluid in the pressurized volume.

The release of second fluid exerts pressures in the existing fracturesexceeding a fracturing threshold of the subterranean formation.

The first and second fluid can be the same fluid. They may also bedifferent fluids with specific characteristics (such as density,viscosity, etc.).

The compressible fluid typically is a gas such as nitrogen.

The release of the second fluid creates a pulse: a wave of excessivepressure/compression propagates in the fluids contained in the confinedvolume and then exerts pressure on the existing fractures.

The pumping equipment at the surface may be smaller than the pumpingequipment for standard hydraulic fracturing as an intermediate volumeacts as a buffer and may accumulate the forces/pressures and releasethem in a fraction of a second.

The fracturing method may enable the extension of a multidirectionalfracture network from an existing initial fracture network. Thisextension may be done in such a way that the curvature radius offractures reorientation toward the preferential fracture plane can becontrolled.

The method may provide pulses with large energy.

The release may also be controlled by parameters in a group ofcomprising at least:

-   -   a maximum pressure exerted on the existing fractures,    -   a duration of the release,    -   a pressure profile exerted on the existing fractures,    -   a time between two consecutive releases.

Thus, each pulse may be adapted to fracture all fractures connected withthe wellbore. The maximum pressure and the duration/rise time of eachpulse may be set so that the fracturing direction for each pulse remainsas close as possible to the existing fracture orientation.

In addition, the time between two consecutive releases may be adjustedso that fractures do not close back.

For fractures existing initially away from the preferential fractureplane, reorientation toward the preferential fracture plane may occur.It may be controlled by the amplitude and rise time of the pressurepulses, as well as the subsequent quick falloff at the end of eachpulse. This control can be further enhanced by the use of high viscosityfluid.

Hence, it is possible to reduce or even avoid the use of proppant(including a lot of chemical additives) and the use of industrial waterfor hydraulic fracturing. Therefore, the proposed method is lesspolluting than the standard hydraulic fracturing.

In a specific embodiment, the method may further comprise:

-   -   /c/ prior to the release of the second fluid, extracting a third        fluid from the confined volume so as to reduce the pressure in        the confined volume below a confined pressure.

The confined pressure may be the pressure of the pressurized volume orany pressure below the pressure of the pressurized volume.

The extraction may be realized through a pumping process.

The third fluid may be either the first fluid or the second fluid or adifferent fluid from the first and second fluids.

Thus, if the pressure of the fluid in the confined volume is below thepressure of the pressurized volume, the release of the fluid from thepressurized volume into the confined volume may create waves ofexcessive-pressure (“hammer effect”).

The confined pressure may be below the predetermined pressure.

In one embodiment, the steps (i.e. /a/ and /b/ or /a/, /b/ and /c/) ofthe method may be executed a plurality of times. The repetition mayinduce an incremental effect with better results than a single powerfulpulse.

The first fluid and second fluid may be a same fluid.

In a specific embodiment, the second fluid may have a higher viscositythan a viscosity of the first fluid.

Viscosity is a physical measure of the resistance of a fluid which isbeing deformed by, for instance, shear stress or tensile stress.

This difference in the fluid viscosity may facilitate the control of thefracturing. The high viscosity fluid (the second fluid) may be pushed bya less viscous fluid (the first fluid).

As the first fluid may be less viscous, it may be easy to pump it fromthe surface into the borehole and may minimize the energy dissipationduring the pumping phase.

As the second fluid is “thicker”, the efficiency of the fracturing maybe improved as the fluid may maintain the fracture open during a longtime.

The second fluid may have a higher density than a density of the firstfluid. Moreover the duration of a pulse is adapted to mitigate fingeringinstability formed at an interface between the first fluid and secondfluid.

A fingering instability may be also known as Rayleigh-Taylorinstability. It is an instability of an interface between two fluids ofdifferent densities, which occurs when the lighter fluid is pushing theheavier fluid.

Moreover, solid particles may be added with the first fluid or thesecond fluid.

These solid particles, injected in the fracture with the fluids, mayhelp the fractures to remain open after the pulse.

In one embodiment, the predetermined pressure may be below thefracturing threshold.

Thus, the pressure in the pressurized volume may be reduced and pumpingmeans at the surface may be downsized.

The invention also relates to a device for extending existing fracturescomprising:

-   -   an upper packer and a lower packer within a borehole, defining a        confined volume with the sides of the borehole, said confined        volume being adapted to be connected to the existing fractures;    -   a pressurized volume located in the wellbore containing a        compressible fluid;    -   pumping means adapted to inject a first fluid in the pressurized        volume so as to pressurize the compressible fluid in said        pressurized volume to a predetermined pressure;    -   a first surface-controlled valve for controlling the first fluid        circulation from the surface to the pressurized volume;    -   a second surface-controlled valve for controlling the second        fluid circulation between the pressurized volume and the        confined volume.

Moreover, the device may further comprise:

-   -   extracting means adapted to pump a third fluid from the confined        volume so as to reduce the pressure in the confined volume below        a confined pressure.

Other features and advantages of the method and apparatus disclosedherein will become apparent from the following description ofnon-limiting embodiments, with reference to the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example, and not by wayof limitation, in the figures of the accompanying drawings, in whichlike reference numerals refer to similar elements and in which:

FIG. 1 a is a schematic overview of a borehole comprising a device forextending existing fractures in a specific embodiment;

FIG. 1 b shows an example of a pumping step of the invention accordingto a given embodiment;

FIG. 1 c shows an example of an extracting step of the inventionaccording to a given embodiment;

FIG. 1 d shows an example of a releasing step of the invention accordingto a given embodiment;

FIGS. 2 a to 2 d details the principle of fingering instability;

FIG. 3 is a second schematic overview of a borehole comprising a devicefor extending existing fractures in another specific embodiment.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 a is a schematic overview of a borehole comprising a device forextending existing fractures in a specific embodiment

In FIG. 1 a, a borehole has been drilled in a subterranean formation100. Fractures 101 exist in this subterranean formation. Inside theborehole, a casing 102 has been installed and is connected to thefracture 101, for instance, thru perforations (not represented).Perforations are made for instance with explosives to blast holes in thecasing and cement sheet

In the casing, different volumes may be defined. A first volume 111,referenced as the pressurized volume, is defined by a cylinder delimitedwith the side of the casing 102, a first plug/disk 103 installed insidethe casing (this first plug/disk 103 may be in the wellhead at the verytop of the well or an additional plug in the wellbore) and an upperpacker 104. A second volume 112, referenced as the confined volume isdefined by a cylinder delimited with the side of the casing 102, theupper packer 104 and bridge plug 105 similar to a lower packer.

Moreover, a first tubing 106 is installed in the casing 102, passesthrough the first plug 103 and has an end in the pressurized volume 111.This first tubing may be used to inject a fluid into the pressurizedvolume 111. To control the injection of the fluid into the pressurizedvolume 111, pumping equipment may be installed at the surface near theborehole. To avoid returns of fluids, a valve 107 is installed in thetubing 106 and may be commanded from the surface or by automatic meansto avoid any flowing of fluid back in the tubing.

A second tubing 108 is also inserted in the casing 102 to allowcommunication of fluids between the pressurized volume 111 and theconfined volume 112. The second tubing and the first tubing may be partof the same tubing. Moreover, in order to ease the assembling and theun-assembling, the second tubing may be mechanically connected to thefirst tubing so that the removal of the first tubing implies the removalof the second tubing. This second tubing may be used to release a fluidfrom the pressurized volume 111 into the confined volume 112. To controlthe release of the fluid into the confined volume 112, a valve 109 isinstalled in the second tubing 108 and may be commanded from the surfaceor by automatic means to avoid any flowing of fluid back in the tubing.

A third tubing 110 is also inserted in the casing 102 to allowcommunication of fluid between the confined volume 112 and the surface.This third tubing may be used to extract a fluid from the confinedvolume 112 in order to reduce the pressure in this volume 112. Tocontrol the extraction of the fluid, a valve 113 is installed in thethird tubing 110 and may be commanded from the surface or by automaticmeans to avoid any flowing of fluid back in the tubing. This thirdtubing and the extraction step is, most of the time, optional.

FIG. 1 b shows an example of a pumping step of the invention accordingto a given embodiment.

During a pumping step according to this embodiment, the valve 107 isopened while the others valves are closed (i.e. 109 and 113). Then,pumping equipment at the surface of the borehole injects a fluid 114into the pressurized volume 111. This injection increases the volume offluids in the pressurized volume and then compresses compressible fluid115 (above the first fluid 114) in this volume 111. Advantageously, thecompressible fluid 115 may be gas (air, nitrogen or other gas).Therefore the pressure in the pressurized volume increases.

The pressurized volume may also contain, prior to the pumping step, aviscous fluid 116 (represented below the injected fluid in the FIG. 1 b)which is more viscous than the injected fluid 114. It stays at thebottom of the pressurized volume 111 as its density is higher than thedensity of the injected fluid 114.

The confined volume may also be filled, prior to the pumping step, withthe viscous fluid 116. An additional tubing (not represented in thisfigure) may be used to inject the viscous fluid 116 in one of thevolumes (111, 112) if the viscous fluid 116 is depleted.

The viscous fluid 116 may be replaced with a “normal” fluid such as theinjected fluid 114.

FIG. 1 c shows an example of an extracting step of the inventionaccording to a given embodiment. This step may be optional.

During an extracting step according to this embodiment, the valve 113 isopened while the others valves are closed (i.e. 109 and 107). This stepaims at reducing the pressure of the confined volume 112 prior to therelease step.

In one embodiment, it is possible to open the valve 113: if the pressurein the confined volume 112 is higher than the hydrostatic pressure ofthe fluid 116 in the tubing 110, a portion of the viscous fluid 116 isextracted through this tubing 110 in the surface direction.

In a different embodiment, it is possible to open the valve 113 and toease the extraction with pumping means at the surface. The pumping meansmay create a depression in the tubing 110 and then extract a portion ofthe fluid 116 in the confined volume.

FIG. 1 d shows an example of a releasing step of the invention accordingto a given embodiment.

During a releasing step according to this embodiment, the valve 109 isopened while the others valves are closed (i.e. 113 and 107). Thus, dueto the difference of pressure in the pressurized volume 111 and in theconfined volume 112, a viscous fluid 116 is released from thepressurized volume into the confined volume. As the valve 109 may befully opened in fractions of second, a wave of excessive-pressure istransmitted in the fluid of the confined volume and then transmitted tothe existing fractures 101 of the subterranean formation 100. Thesepulses exceed the fracturing pressure (or fracturing threshold) of therock and further propagates existing fractures in the formation.

The interface between the viscous fluid and the injected fluid is, inthis embodiment, in the pressurized volume. Nevertheless, it is possibleto set this interface in the confined volume. In such situation, therelease fluid is the normal fluid and not the viscous fluid as describedabove.

FIGS. 2 a to 2 d details the principle of fingering instability. Thefingering instability is also known as the Rayleigh-Taylor instability,or RT instability. This instability is an instability of an interfacebetween two fluids of different densities, which occurs when the lighterfluid is pushing the heavier fluid.

FIG. 2 a details an interface between two fluids 201 and 202, the fluid201 being lighter fluid and the fluid 202 being the heavier fluid. InFIG. 2 a, the interface between these two fluids is plane and no forceis applied on the fluid 201 to push the fluid 202.

A constant force is applied at time t=0 on the first fluid 201 in orderto push the second fluid 202 in the direction—{right arrow over (y)}.Due to the force applied, a complex hydrodynamic effect induces anintrusion of the first fluid 201 into the second fluid 202 (see FIG. 2b, at time t=t₀).

If the force are maintained, this phenomena is amplified (see FIGS. 2 cand 2 d, at time t=2t₀ and t=3t₀) and instability and vortex are created(“mushroom cap”).

It is advantageous to control this intrusion to avoid any instability.Thus, it is possible to limit the duration of each pulse to mitigate thefingering instability formed at the interface between the two fluids.For instance, tests may be conducted on fluids to limit the y-domain ofthe space where the two different fluids may be found at the same time([−0.05; 0.05] for the FIG. 2 b, [−0.12; 0.12] for the FIG. 2 c and[−0.21; 0.21] for the FIG. 2 d).

FIG. 3 is a second schematic overview of a borehole comprising a devicefor extending existing fractures in another specific embodiment.

This embodiment does not provide any tubing for extracting fluid fromthe confined volume 112. In FIG. 1 a, a borehole has been drilled in asubterranean formation 100. Fractures 101 exist in this subterraneanformation. Inside the borehole, a casing 102 has been installed and isconnected to the fracture 101, for instance, thru perforations (notrepresented) in order to ease the fracturing process.

In the casing, different volumes may be defined. A first volume 111,referenced as the pressurized volume, is defined by a cylinder delimitedwith the side of the casing 102, a plug/disk 103 installed inside thecasing (this first plug/disk 103 may be the wellhead at the very top ofthe well or an additional plug in wellbore) and an upper packer 104. Asecond volume 112, referenced as the confined volume is defined by acylinder delimited with the side of the casing 102, the upper packer 104and bridge plug 105 similar to a lower packer.

Moreover, a tubing 301 is installed in the casing 102, has an end in theconfined volume 112. This tubing 301 has been pre-perforated in asection 300 inside the pressurized volume 111 adapted to let the fluidinside the tubing flowing in the pressurized volume (and vice-et-versa).

This tubing may be used to inject a fluid into the pressurized volume111 or in the confined volume 112.

To control the injection of the fluid into the pressurized volume 111,pumping equipment may be installed at the surface near the borehole. Toavoid returns of fluids, a valve 107 is installed in the tubing 301,above the perforated section 300 and may be commanded from the surfaceor by automatic means to avoid any flowing of fluid back in the tubing.

The lower end of the tubing 301 also allows communication of fluidsbetween the pressurized volume 111 and the confined volume 112. Thelower end of the tubing 106 may be used to release a fluid from thepressurized volume 111 into the confined volume 112.

To control the release of the fluid into the confined volume 112, avalve 109 is installed in the lower end of the tubing 301 and may becommanded from the surface or by automatic means to avoid any flowing offluid back in the tubing.

Therefore, during the pumping step, the valve 109 is closed and thevalve 107 is opened so that the fluid passes through the perforations ofthe perforated section 300 from the tubing 301 into the pressurizedzone. During the releasing step, the valve 107 is closed and the valve109 is opened so that the fluid passes through the perforations of theperforated section 300 from the pressurized volume into the confinedvolume.

A person skilled in the art will readily appreciate that variousparameters disclosed in the description may be modified and that variousembodiments disclosed may be combined without departing from the scopeof the invention.

For instance, the device of the invention has been described with threedistinct tubings but other embodiment is possible. A similar device maybe realized with one single tubing and a plurality of valves:

-   -   a single tubing may be inserted in the casing with two openings:        one in the pressurized volume and one in the confined volume;    -   three valves: a first valve controlling the flow through the        opening in the pressurized volume, a second valve controlling        the flow through the opening in the confined volume, and a third        valve in the tubing above the opening in the pressurized volume        and controlling the flow in the tubing.

In this last situation, the pumping step may be realized when the firstand third valves are opened and the second valve is closed. Theextraction step may be realized when the first and second valves areopened and the third valve is closed. The releasing step may be realizedwhen the third and second valves are opened and the first valve isclosed.

The embodiments above are intended to be illustrative and not limiting.Additional embodiments may be within the claims. Although the presentinvention has been described with reference to particular embodiments,workers skilled in the art will recognize that changes may be made inform and detail without departing from the spirit and scope of theinvention.

Various modifications to the invention may be apparent to one of skillin the art upon reading this disclosure. For example, persons ofordinary skill in the relevant art will recognize that the variousfeatures described for the different embodiments of the invention can besuitably combined, un-combined, and re-combined with other features,alone, or in different combinations, within the spirit of the invention.Likewise, the various features described above should all be regarded asexample embodiments, rather than limitations to the scope or spirit ofthe invention. Therefore, the above is not contemplated to limit thescope of the present invention.

1. A method for extending a network of existing fractures in asubterranean formation, comprising: /a/ pumping a first fluid into apressurized volume to pressurize a compressible fluid in saidpressurized volume to a predetermined pressure; /b/ releasing a secondfluid out of the pressurized volume in a confined volume connected tothe existing fractures, the second fluid being released due to arelaxation of the pressurized compressible fluid in the pressurizedvolume; wherein the release of second fluid exerts pressures in theexisting fractures exceeding a fracturing threshold of the subterraneanformation.
 2. The method according to claim 1, wherein the release iscontrolled by parameters in a group of comprising at least: a maximumpressure exerted in the existing fractures, a duration of the release, apressure profile exerted in the existing fractures, a time between twoconsecutive releases.
 3. The method according to claim 1, wherein themethod further comprises: /c/ prior to the release of the second fluid,extracting a third fluid from the confined volume so as to reduce thepressure in the confined volume below a confined pressure.
 4. The methodaccording to claim 3, wherein the confined pressure is below thepredetermined pressure.
 5. The method according to claim 1, wherein thesteps of the method are executed a plurality of times.
 6. The methodaccording to claim 5, wherein the time between two consecutive releasesis adjusted so that fractures don't fully close back.
 7. The methodaccording to claim 1, wherein the first fluid and second fluid are asame fluid.
 8. The method according to claim 1, wherein the second fluidhas a higher viscosity than a viscosity of the first fluid.
 9. Themethod according to claim 1, wherein the second fluid has a higherdensity than a density of the first fluid, and wherein the duration of apulse is adapted to mitigate fingering instability formed at aninterface between the first fluid and second fluid.
 10. The methodaccording to claim 1, wherein solid particles are added with the firstfluid and/or the second fluid.
 11. The method according to claim 1,wherein the predetermined pressure is below the fracturing threshold.12. A device for extending existing fractures comprising: an upperpacker and a lower packer within a borehole, defining a confined volumewith the sides of the borehole; said confined volume being adapted to beconnected to the existing fractures; a pressurized volume located in thewellbore containing a compressible fluid; pumping means adapted toinject a first fluid in the pressurized volume so as to pressurize thecompressible fluid in said pressurized volume to a predeterminedpressure; a first surface-controlled valve for controlling the firstfluid circulation from the surface to the pressurized volume; a secondsurface-controlled valve for controlling the second fluid circulationbetween the pressurized volume and the confined volume.
 13. The deviceaccording to claim 12, wherein the device further comprises: extractingmeans adapted to pump the third fluid from the confined volume so as toreduce the pressure in the confined volume below a confined pressure.